1. Field of the Invention
This invention relates to an apparatus for detecting an initial position of a movable lens in a lens barrel of an inner focusing type for use in optical systems such as cameras.
2. Description of the Background
Conventionally, there have been proposed cameras, e.g., a video camera, which utilize a zoom lens barrel of an inner focusing type. In the inner focusing zoom lens barrel, lenses such as a master lens for focusing, a variator lens for zooming and the like are movable along an optical axis of the lens barrel. In the lens barrel, the relative position of the master lens is determined depending upon a position of the variator lens. Accordingly, it is essential to determine a reference position (origin) of the master lens to enable accurate positioning control. Namely, an initial position of the master lens must be determined as the reference position. For the purpose of fulfilling such requirement, there has been proposed an apparatus for detecting an initial position of the master lens, which includes a photo-interrupter used as a sensor means for detecting the initial position, as disclosed in Japanese Utility Model laid-open appln. No. 2-13216. FIGS. 12 to 14 are directed to the apparatus thus proposed. Referring to FIG. 12, an inner focusing lens barrel 1 has a cylindrical housing 2 which is provided, on front and rear portions thereof, with a focusing lens 3 and a charge coupled device (CCD) 7, respectively. Now, assuming the focusing lens 3 is the front of the lens barrel 1, mounted behind the focusing lens 3 are guide shafts 4a, 4a both of which extend axially parallel to the optical axis of the lens barrel, and in front of the CCD 7 are disposed guide shafts 4b, 4b which also extend parallel to the optical axis. On the guide shafts 4a, 4a is supported a lens holder 6 retaining a variator lens 5 for zooming. The lens holder 6 is reciprocally moved on the guide shafts 4a, 4a by manual operation along an optical axis C of the lens barrel 1. On the guide shafts 4b, 4b is supported a lens holder 9 retaining a master lens 8 for focusing. The lens holder 9 is reciprocally moved on the guide shafts 4b, 4b along the optical axis C by means of a motor M or the like.
The lens barrel housing 2 is also provided with an apparatus 50 for detecting an initial position of the master lens 8. As illustrated in FIG. 13, the apparatus 50 consists of an intercept plate 51 and a photo-interrupter 52 having a light-emitting element 53 and a photo-detecting element 54. The intercept plate 51 is secured to a lower portion of an outer periphery of the lens holder 9. The photo-interrupter 52 is secured at a rear portion of a lower inner-periphery of the housing 2 so as to face the plate 51 upon reciprocating movement of the lens holder 9. The elements 53 and 54 are disposed opposite each other in a direction perpendicular to the optical axis C. When the lens holder 9 moves reciprocally and the plate 51 fixed thereon moves to pass between the elements 53 and 54, the plate 51 intercepts light emitted from the element 53. In response to the interception off the emitted light, the photo-detecting element 54 generates an output signal so that an initial position off the master lens 8 is determined on the basis of a position off the lens holder 9.
Recently, since high miniatuarization off video cameras has been sought, it is necessary to effect highly accurate positioning of movable lenses in the lens barrel. If lens positioning error exceeds approximately 10 .mu.m, incorrect focusing or the like occurs. However, the photo-interrupter 52 of the aforementioned apparatus 50 has an output voltage characteristic varying in response to change in environmental temperature, as shown in FIG. 14. This characteristic causes deterioration of lens positioning accuracy. Moreover, the photo-interrupter 52 is not suitable for miniature cameras due to its large size and further, due to its high manufacturing cost.
In order to overcome such disadvantages in the conventional detection apparatus 50 using a photo-interrupter, there has been proposed an apparatus using a Hall element. Referring to FIGS. 9 to 11, an apparatus 40 for detecting an initial position of a master lens 8 includes a rectangular magnet 41 and a Hall element 42. The magnet 41 consists of north- and south-polar magnet halves 41a and 41b which extend outwardly from a lower outer-periphery of the lens holder 9 and are attached together such that their mating faces extend in a longitudinal direction of the lens holder 9. The Hall element 42 is secured to a lower inner-periphery of the barrel housing 2 so as to face the north-polar magnet half 41a upon rearward movement of the lens holder 9. The hall element 42 detects magnetic flux generated from the magnet 41 and produces an output signal on the basis of an amount of the detected magnetic flux. On the basis of the produced output signal, an initial position of the lens holder 9, i.e., the master lens 8 mounting the magnet 41 is determined. The Hall element 42 has output voltage characteristic as shown in FIG. 10, in which vertical and longitudinal axes indicate the output voltage (V) and distance (D) between the magnet 41 and the Hall element 42, respectively. In FIG. 10, 1/2 Vo indicating half an amount of a peak voltage Vo is detected by a circuit (not shown) connected to the Hall element 42.
However, the output voltage characteristic of the Hall element 42 varies in response to change in environmental temperature, as shown in FIG. 11. Accordingly, an initial position of the lens 8, which is detected upon generation of the initially determined voltage 1/2 Vo, also varies from lo to lo' or lo" according to temperature change so that accuracy in positioning of the lens 8 is deteriorated and thereby focusing error is caused in the camera.